1. Field of the Invention
The present invention relates to an optical fiber component which is used for optical communication and optical information processing. In particular, the present invention relates to an optical filter for improving wavelength dependence of the gain of an optical amplifier containing an Erbium-doped optical fiber (hereinafter called EDFA) so as to perform gain flattening operation and gain equalizing operation. Also the present invention relates to an optical fiber which is used for the optical filter.
2. Description of Related Art
An optical fiber grating which has a periodic refractive index modulation in a longitudinal direction of the optical fiber by utilizing an increase of the refractive index of a Ge-doped section in a silica optical fiber by irradiating with ultraviolet light has advantages such as low inserttion loss, low manufacturing cost, and high reliability. Because of these advantages, the optical fiber grating has been widely used in optical communication.
In general, such optical fiber gratings can be categorized into two types. One is a reflection Bragg grating (hereinafter called reflection FBG) having half a period of light in the medium such as 530 nm of period in a 1550 nm of operational wavelength so as to obtain filtering performance by coupling to a backward direction guided mode light (hereinafter called reflection mode light). A structure of the reflection Bragg grating is shown in FIG. 47.
In FIG. 47, a reference numeral 1 indicates a core. A reference numeral 2 indicates a cladding which is disposed around the core 1. A reference numeral 3 indicates a refractive index increased section which is formed by irradiation with ultraviolet light. A grating section 4 is formed by forming the refractive index increased section 3 in a longitudinal direction of the optical fiber periodically. In the reflection Bragg grating, a guided mode light 5 is coupled to a reflection mode light 6. A transmitted light having rejection loss in the wavelength band is obtained.
Another type of optical fiber grating is a long period grating (hereinafter called LPG) having a period of a several 100s microns and having a transmitting characteristics by coupling a guided mode light to a forward cladding modes. A structure of the LPG is shown in FIG. 48. In FIG. 48, reference numerals 1 to 4 indicate the same features as shown in FIG. 47. The difference between FIGS. 47 and 48 is that a period of the refractive index increased section 3 in FIG. 48 is longer than that in FIG. 47. By doing this, the guided mode light 5 is coupled to the forward cladding mode light 7; thus, a transmitted light having a rejection loss in this wavelength band is obtained.
In a reflection FBG, there are advantages in that it is possible to design loss wavelength characteristics by changing modulation magnitude and period of refractive index in a longitudinal direction of an optical fiber and to realize a large rejection loss such as several −10s dB. In contrast, there are disadvantages in that a spectral smoothness cannot be obtained because ripples of 0.1 to 5 dB exists in the rejection band at about 0.1 nm period and there is a large reflection. For these reasons, the reflection FBG has not been used for a gain equalizer (hereinafter called GEQF) for an optical amplifier using an erbium-doped optical fiber.
On the other hand, in contrast to the reflection Bragg grating, the long-period grating has advantages in that there is no ripple in the rejection band and spectral smoothness is obtained. Also, there is little reflection light. Because of these advantages, the long-period grating has been mainly used for the GEQF. However, in the long-period grating, it is difficult to obtain required loss spectrum. Also, there is a disadvantage in that the obtainable rejection loss is merely 5 dB at best even if an optical fiber for specific use is used. Therefore, the long-period grating is not suitable for gain flattening operation in a broad wavelength bandwidth.
In the near future, it is forecast that an operational wavelength bandwidth may be broadened so as to increase the number of wavelength multiplicity in a wavelength division multiplexing system. Therefore, a GEQF which enables more flexible designing ability for loss wavelength characteristics than that of the long-period grating has been required. According to the above-explained conditions, a GEQF such as a slant optical fiber grating (hereinafter called SFBG) has been developed which has the same advantages as that of the reflection Bragg grating without disadvantages of the reflection Bragg grating. Such GEQF is disclosed in a document of “Gain equalization with optimized slanted Bragg grating on adapted fiber for multichannel long-haul submarine transmission (I. Riant et al, OFC' 99, ThJ6-1, 1999)” (hereinafter called reference document 1).
In FIG. 49, a structure of the slant optical fiber grating is shown. In FIG. 49, reference numerals 1 to 4 indicates the same parts as those indicated in FIG. 47. In FIG. 49, a higher refractive index section 3 is formed such that a surface having equal phase on the grating is slanted to an optical fiber axis A. A grating section 4 is formed by forming the slanted higher refractive index section with the same period periodically as in the case of the reflection Bragg grating.
A direction which is orthogonal to the higher refractive index section 3 is called a grating vector direction of the grating. An angle θ made by the grating vector direction B and the optical fiber axis A is called a slant angle. The slanting degree of the higher refractive index section 3 is indicated by the slant angle. In the slant short period optical fiber grating, a portion of the guided mode light 5 which is reflected by the grating section 4 is coupled to a backward propagating cladding mode 8. In this way, by slanting the grating section 4, the coupling to the backward propagating cladding mode 8, particularly to the LP1x system, is enhanced. Also, by selecting the slant angle θ appropriately, it is nearly possible to surpress the coupling to the reflected mode. At this time, the reflection decreases and multiple reflections of the guided mode do not occur in the light waveguide path. Therefore, unnecessary ripples do not occur. Also, as similar to the short period reflecting optical fiber grating, because it is possible to change filtering characteristics by changing modulation magnitude and period of the refractive index, designing allowability is broadened.
However, because there are a plurality of backward propagating cladding modes 8, it was a general problem in that a loss occurrence wavelength band is broadended. In an optical fiber which is used in a conventional reflection Bragg optical fiber graging and a long period Bragg optical fiber grating, the loss occurrence wavelength band by the mode coupling becomes 20 nm at narrowest. Therefore, it was already found that, in order to manufacture an optical filter having steep loss spectrum or an optical filter having several nm of narrow loss bandwidth, a special optical fiber which can be coupled to a specific cladding mode selectively must be used. This point is described in the reference document 1.
An example of an optical fiber which has been proposed for such an object is shown in FIG. 50. In FIG. 50, a reference numeral 1 is a core and a reference numeral 2 is a cladding. Among the claddings 2, a reference numeral 2a indicates an inner cladding which has photosensitivity and a reference numeral 2b indicates an outer cladding which does not have photosensitivity. In the optical fiber, the refractive index photosensitivity of the core 1 to ultraviolet light is low and the photosensitivity of the inner cladding 2a which contacts the core 1 is high. Also, a non-photosensitive higher refractive index medium is added to the core 1 and a lower refractive index medium is added to the cladding 2 together with Ge.
When a slant optical fiber grating is manufactured by the optical fiber having the above-mentioned structure, the coupling with the reflection mode decreases. Therefore, it is possible to surpress the coupling to the reflection mode by relatively small slant angle. By doing this, it is possible to be coupled to relatively low orders of cladding modes selectively.
By using the optical fiber having the above-explained, simulation for transmission characteristics of the SFBG having 1.1 μm of theoretical cut-off wavelength was performed. Simulation conditions are shown in TABLE 1. The transmission spectrum is shown in FIG. 51. In TABLE 1, the reflection surpressing angle is a slant angle which can surpress the coupling to the reflection mode
TABLE 1Program which is usedApollo Photonics Inc. FOGS-BG ver 2.2aGrating Length1 mmGrating Period530 to 530.35 nmGrating Chirp Ratio0.35 nm/mmSlant AngleReflection Surpressing Angle
As shown in FIG. 51, it is understood that the total width of main loss occurrence bandwidth is 10 nm or narrower.
By the optical fiber having the above-explained structure, it is possible to narrow the loss occurrence area. However, in such a structure, a grating is not formed in the core section in which the optical intensity density is large. Therefore, there are only a few overlapping sections of the light field distribution and the refractive index change; thus, coupling coefficient decreases. Also, a low refractive index medium must be added to the cladding section in addition to Ge. Therefore, usually, less Ge can be added than a case in which only Ge is added. Maximum amount of Ge which can be added is nearly 6 wt %, and it seems that the amount of Ge which can be added should be nearly 5 wt % in order to manufacture an optical fiber in stable mass production. Therefore, there is a limit to increasing the photosensitivity by adding Ge.
For the above-explained two reasons, the rejection loss value which is obtained is not very large. An SFBG was manufactured by using the optical fiber having a structure shown in FIG. 50 and the result thereof is explained as follows.
In FIG. 52, an example of transmission spectrum (fundamental spectrum) of the SFBG which used the above-explained optical fiber is shown. Grating length was nearly 1 mm. According to FIG. 52, it can be understood that the loss occurrence bandwidth is nearly 10 nm. As characteristics of the optical fiber, mode field diameter (hereinafter called MFD) is 10 μm, cut-off wavelength is 1.2 μm, and the Ge-doped amount to the cladding 2 is 4.5 wt %.
The rejection loss area was defined as an area of a main loss occurrence bandwidth as a shaded portion shown in FIG. 51, and an SFBG was manufactured by exposing the optical fiber. Relationship of the exposure time and the occurring loss area is shown in FIG. 53. Here, the loss area is calculated by taking a deteriorating ratio due to an aging process for a purpose of stabilization to a thermal deterioration into account. Here, a saturation loss area is defined as a loss area at the time the increase of the refractive index begins to become saturated, such as a limit in which fitting function can be expressed as S=a·tb. Here, t indicates the exposure time, and s indicates a loss area. In this case, it is understood that the saturation loss area in this case is nearly 3 dB·nm.
For example, a band rejection filter having the transmission rejection bandwidth of 30 nm and rejection loss of 5 dB is manufactured. Such characteristics are an ordinary requirement for the GEQF; thus such a transmission rejection bandwidth is not a wide bandwidth. If it is assumed that 5 dB of loss is formed over 30 nm of bandwidth in a flat manner, a total loss area which is necessary is 150 dB nm. If the maximum grating length is 40 mm, it is understood that nearly 3.8 dB nm of loss area as a fundamental spectrum is necessary.
Actual characteristics of GEQF are not flat; therefore, partially, the loss area having nearly 5.0 dB nm which is 1.3 times as much as the above-mentioned 3.8 dB nm at maximum is necessary to obtain 5 dB of loss. Therefore, in the optical fiber shown in FIG. 50, it is understood that manufacturing the GEQF having 5 dB of peak loss is difficult.
For other methods for increasing the loss, two ideas such as increasing the photosensitivity of the core and decreasing the cut-off wavelength so as to enlarge the optical power in the cladding can be proposed.
If a method such as increasing the photosensitivity of the core is employed, the loss occurrence bandwidth increases; therefore, the photosensitivity should preferably be 20% or less. Thus, this method is not so effective for a purpose of increasing the loss. Also, if a method such as decreasing the cut-off wavelength is employed, there occurs a trade-off in which the loss increases but bending loss occurs too. Therefore, there is a limit in this method. Although it is possible to decrease the limit of cut-off by increasing the relative refractive index difference Δ, in this case, the core diameter decreases. Thus, the loss bandwidth is enlarged. According to the Inventors' analysis, it is believed that the limit of the obtainable saturation loss area which can maintain the loss bandwidth by using the structure shown in FIG. 50 is 4 dB·nm.